a) Field of the Invention
The invention is directed to a variable objective with finite transmission length L from the object plane 0 to the image plane 0xe2x80x2, comprising three axially displaceable lens groups, wherein, successively in the imaging direction, a first lens group has positive refractive power, a second lens group has negative refractive power and the third lens group has positive refractive power.
b) Description of the Related Art
Depending on the imaging task, systems of variable magnification are differentiated into those for finite transmission length, those with infinite distance on one side and those with infinite distance on both sides. Systems of this kind differ from one another with respect to their laws of dynamics and the aimed for parameters as an expression of the magnification and change in magnification. For finite transmission length, the ratio of image height to object height, generally referred to as image scale, is used as magnification. For systems with infinite front focal distance, the focal length is normally indicated. Systems with finite front focal distance and infinite image position use the reciprocal focal length. For afocal systems, as they are called, which border on infinity on both sides, the ratio of the tangential values of the image-side field angle and object-side field angle are indicated.
The basic construction of optical systems with variable magnification for different imaging or scale ratios is known from technical literature and from patent literature.
Objective lenses of the type mentioned above, also known as variable objective systems, must perform two tasks: first, they must change the magnification and, second, they must compensate for focus variations and always image the object sharply on the image position.
The invention is directed to systems with constant finite transmission length. For systems of this kind, at least two axially displaceable lenses or lens groups are always necessary, wherein the magnification is changed with the axial displacement of one lens group, and the second lens group must be displaced simultaneously in such a way that the focusing of the image is maintained with the image position remaining the same. As a result, the displacing movements must be coupled with one another.
Imaging systems of this kind can be used monochromatically or in a very narrow spectral range. For this purpose, it may be sufficient to construct the individual members from one optical medium. When broad spectral regions must be taken into account, it is advantageous to construct the individual optical members achromatically. In the simplest case, a group is formed of two lenses with different dispersions. The term xe2x80x9clens groupxe2x80x9d will also be used synonymously with lens in the following. If a lens is overtaxed, its optical action can be split up between several elements subject to the same dynamics. This splitting up is likewise included within the scope of the invention.
For variable lens systems with finite transmission length, knowledge of the refractive power distribution and the dynamics are sufficient to enable the optical designer to adapt the system to the respective imaging task. Fine-tuning can be carried out with respect to the image field, spectral range, resolution and geometric boundary conditions. The sequence of collective and dispersive parts in the individual groups and the choice of optical media are secondary.
A variable lens system with finite transmission length L is described, for example, in Boegehold, xe2x80x9cDas optische System des Mikroskops [The Optical System of the Microscope]xe2x80x9d, Verlag Technik, Berlin 1958, pages 66ff. The system shown in this reference comprises, considered in the imaging direction, a lens of positive refractive power, a lens of negative refractive power and another lens of positive refractive power. The first and third lenses are fixedly coupled with one another and are displaced jointly. The second lens is arranged so as to be stationary. In order to fully cover the possible change in magnification from xe2x88x920.3 to xe2x88x923.0 in this system, a focus variation of 1.5 mm is indicated. The indicated joint, exclusively linear, movements of the lenses relate to optical compensation.
In addition, there are known systems which compensate mechanically for deviations in image sharpness. There are constructions in which the front or rear lens with positive refractive power is stationary and the remaining highly dynamic elements move relative to one another. A system with less dynamic mechanical compensation is described in DE 43 15 630 A1. In this case, a first lens group with positive refractive power, a second lens group with negative refractive power and a third lens group with positive refractive power are likewise used. The first lens group and third lens group are axially displaceable, but the third lens group carries out a nonlinear movement with respect to the first lens group. The resolution and control of the displacing movement of the components is also carried out via cams. The invention described in the following is associated with this latter type of variable lens system.
It is the primary object of the invention to further develop a variable objective of the type described in the introduction in such a way that the focus variation is also kept small for large transmission lengths L and wide magnification ranges in a simplified construction.
According to the invention, it is provided that the first and third lens groups are displaceable jointly while maintaining the same relative distance L2, wherein the distance L1+D1actual measured between the first lens group and the object plane 0 changes by the amount D1actual=0 . . . D1max, while the distance D2actual between the second lens group and the object plane 0 is subjected to a positively-guided nonlinear change.
In this connection, the movement of the second lens group can be described in a closed formula and can be initiated by simple driving means. This is carried out by the compulsory movement of the second lens group along the function curve of the cosine function depending on the movement of the first and third lens.
According to the invention, the parameters are indicated for the movement of the second lens group which is subject to the following relationship:
D2actual=D2average+C1*cos(C2*D1actual+C3),
where C1, C2 and C3 are command variables. A very good approximation of the exact mechanical compensation of the focus position is achieved in this way.
In various constructions, the principle of the variable objective according to the invention is carried out for transmission lengths in the range of 160 mm to 500 mm.
In this connection, the optical components and the respective maximum possible displacement distance D1max are indicated for transmission lengths L of 160 mm, 200 mm, 240 mm, 280 mm, 320 mm, 380 mm, 450 mm and 500 mm. The link between the displacement distance D1actual=. . . D1max and the change in distance D2actual is given when using the values indicated for each transmission length, D2average, C1, C2 and C3, from the equation mentioned above. The adjusting values for the exact mechanical compensation deviate slightly from the analytically described values.
The displacing movements of the three lens groups can be triggered by different devices. For example, it is possible to control each of the three lens groups separately via stepper motors, wherein the adjusting speeds are to be predetermined such that the above-mentioned conditions are adhered to.
However, the use of cam gear units by which the relative movements between the first lens group and third lens group on the one hand and the second lens group on the other are positively controlled is also advantageous.
In this respect, it can be provided that the first lens group and third lens group are arranged in a fixed manner on a common holder which is displaceable in axial direction and by which the second lens group is coupled via a cam gear unit. The cam gear units are designed in such a way that a guide pin which is fixedly connected with the holder engages in a first control cam and a guide pin which is fixedly connected with the second lens group engages in a second control cam and the distances D1actual and D2actual can be changed depending on the angle of rotation of the control cams about the optical axis.
In alternate constructions, the control cams can be constructed in the wall of a cam tube which is rotatable about the optical axis or at the outer circumference of a cam drum which is rotatable about the optical axis.
A relatively simple mechanical construction can be achieved in that the holder is constructed as a sleeve in which the first lens group and second lens groups are received so as to be fixed with respect to rotation about the optical axis and the outer wall of the sleeve slides in the inner surface of a cam tube in an exact fit.
When the cam tube rotates about the optical axis, the guide pins engaging in the control cams are carried along in axial direction and the movement of the guide pins is transmitted to the first lens group and third lens group and to the second lens group as a displacing movement. The deviation of the displacing movement D1actual of the first lens group and third lens group from the displacement distance D2actual of the second lens group is predetermined by the cam shape and by the inclination of the control cams relative to the circumferential direction of rotation.
The rotational movement of the cam tube or cam drum can be triggered manually or by an electromechanical drive. On the other hand, as possible alternatives, the displacing drive can be constructed in that the holder is provided with a toothed rack in which a pinion engages, the second lens group communicates with an eccentric cam via a sensing lever, and the pinion and eccentric cam are arranged on a common shaft.
As the shaft rotates, the rotational movement of the pinion is transmitted to the toothed rack and is transformed into a longitudinal movement of the first lens group and third lens group. At the same time, the rotation of the shaft is transformed into a displacing movement of the second lens group via a sensing lever, wherein the displacing distance is predetermined by the shape of the eccentric cam.
In another advantageous construction, a cam surface which is oriented substantially parallel to the optical axis and whose contour is sensed by an angle lever which communicates with the second lens group can be provided at the holder. In an axial displacement of the holder or cam, the angle lever is deflected depending on the cam shape and transmits this movement to the second lens group.
The invention will be explained more fully in the following with reference to an embodiment example.